It is generally known that manganese may be leached from manganese dioxide containing ores using sulphur dioxide. However, the sulphur dioxide leaching of manganese dioxide containing materials is also known to produce by-product dithionate ion levels of >5 g/l. These levels may be far higher depending upon the amount of manganese being leached. For example, levels of about 20 g/l are not uncommon. Interestingly, it is reported in Cotton and Wilkinson, Advanced Inorganic Chemistry, 3rd Edition at page 452, that “the method for production of dithionate or dithionic acid is the reaction between sulphur dioxide or sulphite with manganese dioxide in the presence of acid”.
Low grade manganese dioxide feedstock (<40% Mn) are presently uneconomic to process using conventional roast-reduction and sulphuric acid leaching to produce manganese sulphate. High grade ores (>40% Mn) are needed to justify the economics of the roast reduction process. Presently, all leaching of manganese dioxide containing materials using sulphur dioxide leads to the formation of >5 g/l levels of dithionate ions in solution. With dithionate ion levels of this magnitude it is generally necessary to incorporate into any flow sheet a high capital cost stage, being “oxidation” or “aging”. The long residence times required to “oxidise” the dithionate ion from the >5 g/l levels down to lower than 1 g/l are highly capital intensive.
Failure to control dithionate levels in the production of a manganese sulphate crystal product has previously led to the manganese dithionate contaminant in that product slowly reacting to release sulphur dioxide gas.
It would prove advantageous to provide a process whereby low-grade manganese dioxide containing materials or feedstock could provide manganese sulphate leach solutions with a level of dithionate ion less than about 5 g/l, and preferably less than 1 g/l.
The ability to recover manganese dioxide from low-grade feedstocks will avoid or at least reduce the need for further manganese ore mining and land disturbance, bringing various environmental benefits. For example, the utilisation of manganese tailings allows for conservation of existing resources.
Further, the use of a hydrometallurgical route for the reduction of Mn(IV) negates the need for the use of gas fired kilns or fluid bed reactors, feed stocks no longer need to be heated to about 1000° C. and then cooled prior to leaching, and there is lesser need for carbon input, which in turn results in lower greenhouse gas emissions.
Still further, the use of the relatively easily controlled hydrometallurgical route allows monitoring of the solution potential of the leach solution or slurry thereby indicating complete dissolution of Mn(IV). The use of the sulphur dioxide leach provides complete conversion of Mn(IV) to Mn(II), thereby avoiding the production of leachable manganese species in solid residues.
In particular, if it is desired to produce electrolytic manganese dioxide (“EMD”), solutions containing elevated dithionate ion levels result in chemical reactions occurring that effect the quality and purity of the EMD produced in the electrowinning cells. Also, hydrogen sulphide is evolved, bringing with it certain occupational health and environmental issues.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.
Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.